The present invention relates to a method for reciprocally scanning an original and an apparatus therefor.
Conventionally, there is known a copying method in which original and an exposure optical system are moved relative to each other for performing reciprocal scanning of the original, while at the same time the surface of a photoconductor is moved in a predetermined direction, whereby the optical image of the original is projected onto the photoconductor through different optical paths during the forward scanning and the backward scanning. For example, see Japanese Laid-open patent application Ser. No. 54-070,047.
By the above-mentioned copying method, the copying efficiency can be maximized.
In the case of the above-mentioned copying method, there will be little problems so long as an original scanned is the maximum size that can be copied by the copying machine. However, when an original of a smaller size than maximum is employed, there are several problems, as will now be explained.
Before discussing such problems, however, the longitudinal direction and the transverse direction of copying in the above-mentioned copying method are defined for the sake of convenience of explanation as follows: The longitudinal direction is the direction parallel to the original scanning direction, which is also parallel to the movement direction of the optical image of the original in the exposure section. Furthermore, the original scanning direction is parallel to the movement direction of the surface of the photoconductor and to the transportation direction of the recording sheets. Therefore, not only the original scanning direction, but also the movement direction of the optical image of the original, the movement direction of the surface of the photoconductor and the transportation direction of the recording sheets, are longitudinal.
The transverse direction is the direction normal to the longitudinal direction. Therefore, the transverse direction exists on the surface of the original to be scanned. As in the case of the longitudinal direction, with respect to the movement direction of the surface of the photoconductor and the movement direction of recording sheets, there exist respective transverse directions. In those cases, those transverse directions exist on the surface of the photoconductor and on the surface of each recording sheet.
In this specification, the term recording sheet is used to collectively cover photosensitized sheets for use in direct copying processes, such as the electrofax process, and image transfer sheets for use in a visible image transfer process or in an electrostatic latent image transfer process.
Throughout this specification, when it is unnecessary to distinguish the photosensitized sheet from the image transfer sheet, the term "recording" will be used, while when it is necessary to distinguish them, the the words "photosensitized sheet" and "image transfer sheet" are separately used.
In the aforementioned original reciprocating scanning method, the original is scanned in opposite directions, i.e., by a forward movement and by backward movement: In contrast to this, the movement direction of the surface of the photoconductor is constant. Therefore, in order to achieve the proper exposure of the photoconductor regardless of the original scanning direction, an in-prism lens or a roof mirror has to be disposed in one of the two optical paths for exposure for the forward scanning use and the backward scanning use. Hereinafter, the optical path for exposure employing the in-prism lens or the roof mirror is referred to as a roof type optical path.
Furthermore, together with the above-mentioned definition of the recording sheet, the term photoconductor covers a photoconductor sheet in this specification.
Referring to FIG. 1A, reference symbol F designates an original stacking surface. The size of the original stacking surface is equal to the maximum size of the original that can be copied. Reference symbol L designates the longitudinal direction, and symbol T the transverse direction. For the sake of convenience of explanation, the following copying system is assumed. The photoconductor is formed in the shape of a drum and is rotated in a predetermined direction and is exposed during the forward original scanning. An electrostatic latent image formed on the surface of the photoconductor by the exposure is developed to form a visible image. That visible image is transferred from the surface of the photoconductor to a recording sheet. Further for convenience of explanation, the peripheral length of the drum-shaped photoconductor (hereinafter referred to as the photoconductor drum) is equal to the length of the original stacking surface in the longitudinal direction thereof, and the forward original scanning is performed from an end A.sub.1 to an end B.sub.1 of the original stacking surface. Upon the completion of the forward original scanning, the backward original scanning is performed in the direction opposite to the forward original scanning. In FIG. 1A, arrows C.sub.1 and C.sub.2 respectively designate the forward original scanning direction and the backward original scanning direction.
Furthermore, in FIG. 1A, reference symbol H designates the developed surface of the photoconductor surface, and symbol D designates the movement direction of the surface of the photoconductor drum.
As shown in FIG. 1B, it is assumed that a small original O is placed on the original stacking surface F. In FIG. 1B, the back side of the original O can be seen and, on the front side of the original O, an image I.sub.m is formed. When the forward original scanning is performed under this condition and the photoconductor drum is exposed to the optical image of the original O, the end A.sub.1, which is the initiation line of the forward original scanning, corresponds to the exposure initiation line .alpha. of the photoconductor drum. Therefore, when an electrostatic image thus formed is developed, a visible image I.sub.m1 is formed on the surface H of the photoconductor drum on the side of the exposure initiation point R on the developed surface of the drum. When the forward original scanning is successively performed, the end B.sub.1, which constitutes the initiation point of the backward original scanning, corresponds to the exposure initiation point .alpha.. Therefore a visible image I.sub.m2 is formed on the opposite side with respect to the exposure initiation point .alpha.. The visible images I.sub.m1 and I.sub.m2 are transferred to the recording sheet.
However, the positions of those images are different on the surface of the photoconductor drum. Therefore, if the visible image I.sub.m2 is transferred to the recording sheet with the same timing as that of the visible image I.sub.m1, the image transfer would not be performed properly. That is, when the recording sheet and the original are the same size, the visible image I.sub.m2 and the recording sheet are in completely different positions. As a result, the visible image I.sub.m2 cannot be transferred to the recording sheet at all.
Thus, in the case where the exposure of the photoconductor drum is performed during the backward original scanning and the original is not the maximum original, it is required that the recording position of the copy image and the position of the recording sheet be suitably adjusted.
The case where an original O, which is smaller than the maximum size original in the transverse direction, is placed on the original stacking surface F.sub.1 as shown in FIGS. 2A and 2B will now be discussed, using the above-mentioned copying system. In the following description, it is assumed that the copy image and the recording sheet are properly adjusted with respect to their position.
In FIGS. 2A and 2B, reference numeral 8 designates an image formation lens system. For the sake of convenience of explanation, it is assumed that the image formation lens system 8 is a through-lens. The image formation lens system 8 is shared by the forward original scanning optical path and the backward original scanning path. The backward original scanning path is a roof-type optical path.
When the forward original scanning is performed in the direction of arrow C.sub.1 under the conditions as shown in FIG. 2A, the end portion A.sub.1 in the longitudinal direction on the original stacking surface F and the end portion P in the transverse direction thereof correspond to an exposure initiation point .alpha. and a side end portion .beta. on the surface of the photoconductor. Therefore, the visible image I'.sub.m1 corresponding to an image I.sub.m' on the original O.sub.1 is formed as shown in FIG. 2A, and when a recording sheet S is transported in the transverse direction with the end side P as its transportation reference, the position of the visible image I'.sub.m1 and that of the recording sheet S, become different as shown in FIG. 2A. As a result, the visible image I'.sub.m1 is not properly transferred to the recording sheet S and part of the visible image I.sub.m' remains untransferred.
In contrast to this, when the backward original scanning is performed as shown in FIG. 2B, since the optical path is of a roof type, the end portion P in the trasverse direction on the original stacking surface F corresponds to an end portion .gamma. in the transverse direction on the surface H of the photoconductor drum.
Therefore, a visible image I.sub.m2 is formed in the position shown in FIG. 2B, so that the positional relationship between the visible image I.sub.m2 and the recording sheet S is properly set in the transverse direction, whereby the proper transfer of the visible image is performed. Thus, in the copying system of the type where the exposure of the photoconductor is performed during the forward original scanning, it is required that the positional relationship between the copy image and the recording sheet be adjusted in the transverse direction as well with respect to the recording position, when copies are made from an original other than the maximum size original.
Therefore, such necessity for adjustment of the positional relationship between the copy image and the recording sheet is a problem to be solved in the conventional copying system.
In this specification, when the recording sheet is a photosensitized sheet, the term "recording position" means the position where exposure is done, while, when the recording sheet is an image transfer sheet, it means the position where an electrostatic latent image or a visible image is transferred to the the image transfer sheet.
As regards the improper positional relationship between the image to be copied and the recording sheet in the longitudinal direction and the transverse direction, the improper positional relatinship in the transverse direction can be solved by the following procedure: When copies are made from originals except the maximum size original, a smaller original is placed on the original stacking surface in such a manner that the center of the original in the transverse direction is positioned in conformtity with the center of the original stacking surface in the transverse direction thereof, and the recording sheet corresponding to that original is transported in such a manner that the center of the recording sheet in the transverse direction is positioned in the center of the copying machine in the transverse direction thereof. The above-mentioned original stacking method and the recording sheet transporation are respectively referred to as the center-reference original stacking and the center-reference recording sheet transportation.
In contrast to this, an original stacking method in which the original is placed on the original stacking surface in such a manner that one side portion of the original is in conformity with one side of the original stacking surface in the transverse direction thereof is referred to as one-side-reference original stacking. A recording sheet transportation method in which the recording sheet is transported, using one side portion in the transverse direction, is referred to as one-side-reference recording sheet transportation.
The copying method shown in FIGS. 2A and 2B is of one-side-reference original stacking and one-side-reference recording sheet transportation, and their reference is one and the same and is located on side P.
In center-reference original stacking and the center-reference recording sheet transportation, the exposure is performed in the same central portion in the transverse direction during both the forward scanning and the backward scanning, regardless of the size of original. Therefore, there is no shift between the original optical image and the recording sheet in the recording position, so that the previously mentioned problems do not take place.
However, when a copying apparatus of the forward-and-backward-original-scanning type is designed in practice, the center-reference original stacking and the center-reference recording sheet transportation are not always adopted in combination. There may be a case where the one-side reference original stacking and the center reference recording sheet transportation have to be adopted in combination or there may be a case where the one-side-reference original stacking and the one-side-reference recording sheet transportation have to be adopted in combination. In any of these combinations, the above-mentioned problems take place.